Most liquid medicines and the like that are typically injected with a syringe have a sufficiently low viscosity that the plunger of the syringe can be manually compressed into its barrel, to expel the liquid therefrom, with relatively little effort. However, when a high viscosity material is to be injected, particularly through a hypodermic needle having a small opening, difficulties are presented. As the force required to move the plunger increases, the degree of control that can be exercised over the rate of injection is lessened. For materials having extremely high viscosity, such as a collagen implant for example, the force required to move the plunger approaches the maximum force available to finger and thumb muscles that is consistent with fine motor control.
In the past, most devices for assisting the compression of a syringe have been relatively complex and often obstruct the view of the needle or otherwise pose difficulties relative to conventional injection techniques. One type of device comprises a pistolgrip pawl and ratchet mechanism. A drawback associated with this type of device is that the advance of the plunger is limited to steps of a fixed size, as determined by the ratchet mechanism. Consequently, the degree of control that is available is limited. In addition, most commercially available devices of this type are adapted for relatively large syringes, and are not suited for use with smaller syringes having a capacity of 1-3 cc., for example.
Other types of devices are not suited for use with conventional syringes. For example, one pneumatic syringe assist device requires a syringe that does not have a plunger, so that the compressed air can act directly on the material to be injected.